Pressure sensor with integrated cooler and methods of using

ABSTRACT

The present invention relates to a method of determining both pressures and temperatures in a high temperature environment. The present invention also relates to a method of determining temperatures about a pressure-sensing element using a bi-functional heater. In addition, the present invention preferably relates to a pressure sensor with the pressure-sensing element and a heating element both integrated into the sensor&#39;s packaging, preferably onto the diaphragm of the pressure sensor, and particularly to such a pressure sensor capable of operating at high or elevated temperatures, and even more particularly to such a pressure sensor wherein the heating element is capable of both heating, at least in part, the pressure-sensing element and monitoring the temperature of the application area. Preferably, the pressure-sensing element is formed from shape memory alloy (SMA) materials that can be used at high or elevated temperatures as a pressure sensor with high sensitivity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/820,403 filed Jun. 19, 2007, now U.S. Pat. No. 7,415,884; which is acontinuation of U.S. patent application Ser. No. 11/226,806 filed Sep.14, 2005 that issued as U.S. Pat. No. 7,258,015; which is a continuationin part of U.S. patent application Ser. No. 10/666,156 filed Sep. 19,2003 now abandoned; which is a continuation in part of U.S. patentapplication Ser. No. 09/726,257 filed Nov. 30, 2000 that issued as U.S.Pat. No. 6,622,558.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of determining both pressuresand temperatures in a high temperature environment. The presentinvention also relates to a method of determining temperatures about apressure-sensing element using a bi-functional heater. In addition, thepresent invention preferably relates to a pressure sensor with thepressure-sensing element and a heating element both integrated into thesensor's packaging, preferably onto the diaphragm of the pressuresensor, and particularly to such a pressure sensor capable of operatingat high or elevated temperatures, and even more particularly to such apressure sensor wherein the heating element is capable of both heating,at least in part, the pressure-sensing element and monitoring thetemperature of the application area. Preferably, the pressure-sensingelement is formed from shape memory alloy (SMA) materials that can beused at high or elevated temperatures as a pressure sensor with highsensitivity.

2. Technical Background

In recent years there has been a need for high or elevated temperaturepressure sensors for various applications including for use in harshenvironments. In these harsh environments such as for use in enginecylinders and turbine engines, the pressure sensors are exposed tocorrosive, oxidizing environments, which put high mechanical and thermalstresses on the sensors. Various approaches have been taken in order toprotect the pressure sensors from these environmental conditions and toallow the sensor to remain operational over extended periods of time.These approaches include sealing the pressure sensor to shield it fromthe environment.

While sealing the sensor from the environment has helped create a moredurable sensor, at high temperatures these sensing devices also sufferfrom the drawback of having too low of a gage factor resulting insensors with larger diaphragms or sensors with signals that aredifficult to measure. Gage factor is a measure of the sensitivity of thesensor. With too low of a gage factor, the sensitivity of the sensingelement is reduced creating difficulty in reading the sensing element,or the diaphragm size has to be increased to make up for the reducedsensitivity. These sensors are typically manufactured by diffusing thesensing elements into a silicon diaphragm. With these types of sensorsthe gage factor significantly decreases with increasing temperature.Another drawback of these types of sensors is that the sensor issubjected to thermal variations at the point of application of thesensing element resulting in proportionally large variations in thesignals received from the sensor. Both these drawbacks have beenaddressed by providing a means of cooling these types of sensors inorder to maintain a higher level of sensitivity or gauge factor, andless variability. Cooling the sensor, however, is not desirable becauseof the cost, complexity and space requirements for such devices. Coolingthe sensor further sets up a large temperature gradient between thesensing device and the application environment, causing additionalproblems.

What is needed is a pressure sensor with a high sensitivity at elevatedtemperatures that is constructed to reduce or eliminate thermalvariations about the sensing element at the point of application. Whatis also needed is a pressure-sensing device that can further monitorapplication temperatures. It is therefore the object of the presentinvention to provide a high temperature pressure sensor without thedrawbacks of the prior art. It is further an object of the presentinvention to provide a method of determining both pressures andtemperatures of a high temperature environment. It is still further theobject of the present invention to provide a high temperature pressuresensor with an integrated heating element on the diaphragm to helpeliminate thermal variations at the point of application. It is stillfurther an object of the present invention to provide a pressure sensorwith an integrated bi-functional heating element. It is still furtherthe object of the present invention to provide a pressure sensor with asmaller sized diaphragm with an integrated heating element on thediaphragm, which is also capable of reading higher pressures. It isstill further the object of the present invention to provide a heatingelement, which can be used to measure the temperature of the applicationarea, as well as to control itself. Finally, it is even still furtherthe object of the present invention to provide a high temperature sensormade from a shape memory alloy (SMA) material with a heating elementintegrated into the packaging.

SUMMARY OF THE INVENTION

The present invention relates to a method of determining both pressuresand temperatures in a high temperature environment. The presentinvention also relates to a method of determining temperatures about apressure-sensing element using a bi-functional heater. In addition, thepresent invention preferably relates to a pressure sensor with thepressure-sensing element and a heating element both integrated into thesensor's packaging, preferably onto the diaphragm of the pressuresensor, and particularly to such a pressure sensor capable of operatingat high or elevated temperatures, and even more particularly to such apressure sensor wherein the heating element is capable of both heating,at least in part, the pressure-sensing element and monitoring thetemperature of the application area. Preferably, the pressure-sensingelement is formed from shape memory alloy (SMA) materials that can beused at high or elevated temperatures as a pressure sensor with highsensitivity.

The method of determining both pressures and temperatures in a hightemperature environment, preferably, uses the pressure-sensing elementto determine the pressure and the heating element not only to regulatethe temperature of the pressure-sensing element but to act as thetemperature sensing element as well. The bi-functional heating elementpreferably is a resistive heating element. The present invention wouldalso work as well with a bi-functional cooling element, such as abimetal cooler and preferably a Peltier cooler. The method of thepresent invention can be used in various applications such as marine,aerodynamic, diesel, electric power generation, process control and thelike where pressure and in some instances temperature are importantcharacteristics of the application. The pressure and temperature sensingcapabilities can then be used to control, adjust or regulate variousinput parameters of the application process such as the fuel/airmixture, valve settings, injection timings or thermal regulators. Themethods of the present invention allows for greater fuel efficiency aswell as reduced emissions from combustion processes and greaterefficiency and safety in chemical processes.

The heater element and pressure-sensing element of the pressure sensorare integrated into the pressure sensor packaging and preferablytogether onto the pressure sensor diaphragm. The heater element and thepressure-sensing element being configured to prevent short circuitingbetween the heating and pressure-sensing elements, and to allow theheater to maintain stable thermal characteristics of thepressure-sensing element, preferably similar to the applicationenvironment. Depending on the characteristics of the material, thicknessand shape of the diaphragm, or the application, the heater can bepositioned on the opposite side of the diaphragm from the sensingelement, positioned adjacent to the sensing element, positioned above orbelow the sensing element (but separated by a dielectric layer) or inany other position relative to the sensing element that provides stablethermal characteristics during the application or use of the pressuresensor.

The heater element can be controlled to maintain a stable thermalenvironment, with temperatures at or around the application temperature,for the pressure-sensing element using another sensor to sense thetemperature and a controller to receive signals from this sensor andadjust the electrical input into the heater element to achieve ormaintain thermal stability about the pressure-sensing element.Alternatively, with the proper heater element having thermally sensitiveresistive characteristics, the temperature at the heater element can bedetermined by the electrical requirement characteristics of the heaterelement to determine the temperature and thereby through a controller tomaintain the thermal stability about the pressure-sensing element.

In a number of embodiments, the sensor of the present inventioncomprises a substrate material, a flexible diaphragm provided on thesubstrate material and a sensor member deposited on the flexiblediaphragm. The sensor member or pressure-sensing element may be formedfrom a thin film SMA material and is capable of undergoing a phasetransformation, such as from its austenite phase to its martensitephase, in response to a physical stimulus, such as strain, being appliedthereto. During such a phase transformation, the electrical resistanceof the thin film SMA material undergoes a substantial change. Thischange in electrical resistance can be correlated to a change in strainbeing applied to the thin film material. In this manner pressure can bemeasured. The present invention also provides a method for measuring aphysical stimulus comprising the steps of providing a sensor comprisinga thin film SMA material; measuring a physical property, such as theelectrical resistance, of the thin film SMA material immediately beforethe material undergoes a phase transformation caused by the applicationof a physical stimulus thereto; applying a physical stimulus to the thinfilm SMA material causing the material to undergo a phasetransformation; measuring a physical property, such as the electricalresistance, of the thin film SMA material immediately after the materialundergoes a phase transformation; determining the difference in thevalue of the physical property, e.g., the electrical resistance, thatoccurs during the phase transformation; and utilizing the difference inthe value of the physical property to determine the magnitude of thephysical stimulus being applied to the thin film SMA material.

One embodiment of the present invention includes a method of determiningboth pressures and temperatures in an elevated pressure chambercomprising the steps of providing a pressure sensing element for anelevated temperature in which a pressure and a temperature are to bemeasured; providing a heating element in or about the elevated pressurechamber in which a pressure and temperature are to be measured;measuring the pressure of the elevated pressure chamber through thesensing element; and measuring or predicting the temperature of theelevated pressure chamber through the heating element.

In another embodiment, the present invention includes a method ofdetermining both pressures and temperatures in an elevated pressurechamber comprising the steps of providing a sensing element for anelevated pressure chamber in which a pressure and a temperature are tobe measured; providing a heating element in or about for the elevatedpressure chamber in which a pressure and temperature are to be measured;heating the sensing element, at least in part, with the heating element;measuring the pressure of the elevated pressure chamber through thesensing element; and measuring or predicting the temperature of theelevated pressure chamber through the heating element.

In still another embodiment, the present invention includes a method ofadjusting an engine comprising the steps of providing a sensing elementfor an engine chamber in which a pressure and a temperature are to bemeasured; providing a heating element for the engine chamber in which apressure and temperature are to be measured; heating the sensingelement, at least in part, with the heating element; measuring orestimating the pressure of the engine chamber through the sensingelement; and adjusting parameters for the engine chamber based in parton the measured or estimated pressure.

In yet another embodiment, the present invention includes a method ofcontrolling a temperature about a pressure-sensing element comprisingthe steps of providing a pressure-sensing element for an elevatedtemperature application in which a pressure is to be measured; providinga heating element with a power input; heating the pressure-sensingelement, at least in part, with the heating element; determining orestimating the temperature of about the pressure-sensing element bymeasuring at least one electrical characteristic of the heating element;and adjusting the power input to the heating element in response to themeasured electrical characteristic or characteristics of the heatingelement.

In yet another embodiment, the present invention includes a pressuresensor comprising a sensing element; a diaphragm having an upper and alower surface; and a resistive heating element capable of working as aheating element and a thermistor.

In yet another embodiment, the present invention includes a pressuresensor comprising a sensing element; a diaphragm having an upper and alower surface; and a resistive heating element wherein the resistiveheating element maintains the temperature of the sensing element at orabove the operating temperature range of the pressure sensor and is usedto measure the operating temperature of the pressure sensor.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hysteresis curve of Electrical Resistance or Length Changeor Volume Change vs. Temperature for a hypothetical SMA material.

FIG. 2 is a graph of Electrical Resistance vs. Temperature and shows thehysteresis curve of a thin film SMA material under three differentstrain levels.

FIG. 3 is a graph of Electrical Resistance vs. Loading Condition for athin film SMA s material subjected to an increasing strain level from0-1.2%.

FIG. 4 is a Best Fit Line Graph of Electrical Resistance vs. Strainshowing the linear response there between for a thin film SMA material.

FIG. 5 is a plan view of a thin film SMA material strain sensor.

FIG. 6 is a cross-sectional view of a thin film SMA material strainsensor taken across section-indicating lines 6-6 of FIG. 5.

FIG. 7 is an electrical schematic of the SMA material in a WheatstoneBridge configuration.

FIG. 8 is a plan view of a thin film SMA material strain sensorincluding a temperature measuring element and a heating element.

FIG. 9 a) is a plan view of another embodiment of the pressure-sensingelement and the heating element incorporated on the same substrate and 9b) is a cross-sectional view of the same embodiment.

FIG. 10 a) is a plan view of another embodiment of the pressure-sensingelement and the heating element incorporated on the same substrate and10 b) is a cross-sectional view of the same embodiment.

FIG. 11 a) is a plan view of still another embodiment of thepressure-sensing element and the heating element incorporated on thesame substrate and 11 b) is a cross-sectional view of the sameembodiment.

FIG. 12 a) is a plan view of the top surface another embodiment of thepressure-sensing element, 12 b) is a cross-sectional view, and 12 c) isplan view of the bottom surface of the substrate holding thepressure-sensing element.

FIG. 13 is a cross-sectional view of an embodiment of thepressure-sensing element and the heating elements incorporated onseparate substrates.

FIG. 14 is a cross-sectional view of another embodiment of thepressure-sensing element and the heating elements incorporated onseparate substrates.

FIG. 15 is a block diagram of one embodiment of the electrical circuitryfor operating the temperature control and measurement with thebi-functional sensing element.

FIG. 16 is a block diagram of another embodiment of the electricalcircuitry for operating the temperature control and measurement with thebi-functional sensing element.

FIG. 17 is a cut-away schematic illustration of a portion of oneembodiment of the pressure sensor of the present invention mounted in apressure chamber where heat flux is into the sensing element from thechamber.

FIG. 18 is a cut-away schematic illustration of a portion of oneembodiment of the pressure sensor of the present invention mounted in apressure chamber where heat flux is into the chamber from the sensingelement.

FIG. 19 is a graph comparing the gage factors of other prior artpressure sensors with the pressure sensor of the present invention overa wide temperature range.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a method of determining both pressuresand temperatures in a high temperature environment. The presentinvention also relates to a method of determining temperatures about apressure-sensing element using a bi-functional heater. In addition, thepresent invention preferably relates to a pressure sensor with thepressure-sensing element and a heating element both integrated into thesensor's packaging, preferably onto the diaphragm of the pressuresensor, and particularly to such a pressure sensor capable of operatingat high or elevated temperatures, and even more particularly to such apressure sensor wherein the heating element is capable of both heating,at least in part, the pressure-sensing element and monitoring thetemperature of the application area. Preferably, the pressure-sensingelement is formed from shape memory alloy (SMA) materials that can beused at high or elevated temperatures as a pressure sensor with highsensitivity.

The method of determining both pressures and temperatures in a hightemperature environment, preferably, uses the pressure-sensing elementto determine the pressure and the heating element not only to regulatethe temperature of the pressure-sensing element but to act as thetemperature sensing element as well. The bi-functional heating elementpreferably is a resistive heating element. The present invention wouldalso work as well with a bi-functional cooling element, such as abimetal cooler and preferably a Peltier cooler.

The method of the present invention can be used in various generalapplications including but not limited to marine, aerodynamic, diesel,electric power generation and the like where pressure and in someinstances temperature are important characteristics of the applicationor process. These pressure sensors can specifically be used on test &measurement equipment, stationary and marine diesel, off-road diesels,over-the-road diesels, automotive (combustion) engines, aircraftturbojets (commercial and military), marine turbojets (commercial andmilitary), fixed turbines for power generation, boilers and processingequipment, and the like. The pressure and temperature sensingcapabilities can then be used, in most if not all of these applications;to control, adjust or regulate various input parameters of theapplication process such as the fuel/air mixture. The methods of thepresent invention further preferably allow for greater fuel efficiencyas well as reduced emissions from these various processes.

Preferably, the pressure sensor measures or estimates the pressure inthe application environment by measuring the amount of deflection of adiaphragm caused by a pressure in the chamber being measured. Morepreferably, this is done by placing a strain gauge on the diaphragm toaccurately measure the deflection at the surface. More preferably, thestrain gauge is made from a SMA material whose phase transformationtemperature range is at or about the application temperature of theapplication in which the pressure sensor is to be used.

The pressure chamber referred to in various embodiments of the presentinvention can be for example a cylinder for a combustion or a dieselengine, a can for a turbine engine, a boiler or parts of a boiler, achemical reaction vessel, a pressurized fluid vessel including varioushydraulic systems, and the like. These examples are given by way ofdemonstration not limitation.

The temperature control or heating element can be fully integrated intothe sensor packaging, or it can be a separate element that is thermallycoupled to the pressure-sensing element. More preferably, thetemperature control or heating element is at least partially integratedonto the diaphragm of the pressure sensor along with thepressure-sensing element. The temperature control or heating element ispreferably a bi-functional heating or cooling element, and morepreferably a bi-functional heating element. One of the functions of theheating or cooling element is to heat or cool the pressure-sensingelement. The other function is to measure the temperature about thepressure-sensing element and/or to measure the application temperature.

The bi-functional heating and cooling element can measure these localtemperatures in a number of ways known to those skilled in the art. Oneof these-methods involves the heating element being a resistive heater.For a cooled sensor the methods are similar, but with a sign change. Inthis first method, where there is a heat flux into the sensor, theheating element is intermittently turned off and the substrate on whichthe heating element is mounted is allowed to come to thermal equilibriumwith the surroundings. The resistance of the heating element ismeasured. This is compared to a look-up table or equation held in theprocessing part of the pressure sensor or controller used with thepressure sensor, and the temperature is identified as a function,ideally of linear resistance. In another method, the temperature of thepressure sensor is maintained at some constant, warmer than theapplication environment, such that small amounts of heat are transferredinto the application environment. The amount of power that is requiredto maintain the temperature of the pressure sensor is a function of thethermal transfer characteristics and the temperature difference betweenthe pressure sensor and the environment. Use of the bi-functionalheating element in this mode requires knowledge of the thermal transferpaths and mechanisms of the pressure sensor. The power requirements canthen be directly related to the temperature difference between thepressure sensor and the application environment.

In a number of embodiments, the present invention relates to a methodand sensor to detect strain utilizing the strain-dependent electricalresistance effect of SMA materials near their phase transformationtemperature. The strain can be produced by any external stimulus, suchas mechanical vibration, pressure, force, stress or other straininducing external input.

A number of embodiments of the present invention include a pressuresensor comprised of a substrate with an opening, and a flexiblediaphragm held across the opening of the substrate. The substrate can bemade from any material known to those skilled in the art. The opening ofthe substrate is important. If the opening is large it correspondinglyrequires a larger pressure-sensing device, and in the case ofmeasurement of larger pressures a diaphragm with increased mechanicalproperties. If the opening is small, the sensitivity of the devicesuffers. Therefore with smaller devices, and therefore smaller openings,it is desirable to have pressure sensors with the highest possible gagefactor (which is explained below). Preferably, the maximum dimension ofthe opening of the substrate across which the flexible diaphragm lies isless than about 1.0 mm, more preferably less than about 0.5 mm, and mostpreferably less than about 0.25 mm. The diaphragm, likewise, can also bemade from any material known to those skilled in the art, provided ithas a flexibility corresponding to the pressure and the pressure rangethat is desired to be measured. Preferably, the diaphragm is made from amaterial on which the sensor can be applied or deposited and a good bondcan be formed. More preferably, the diaphragm is made from silicon.Preferably, the flexible diaphragm has a thickness less than about 350um extending across the opening of the substrate, more preferably lessthan about 250 um, even more preferably less than about 225 um, stilleven more preferably less than about 150 um, and most preferably lessthan about 50 um.

The pressure sensor of the present invention preferably is capable ofmeasuring a wide range of pressures, and for measuring fairly highpressures without premature failure. This is because while thesepressure sensors can be used for any application known to those skilledin the art, many of those applications are in areas requiring themeasurement of fairly high pressures. Two areas of prime application ofthese pressure sensors are for the measurement of pressures in turbineengines as well as the measurement of pressures in internal combustionengines. Because of this, preferably, the pressure sensor is capable ofmeasuring pressures above 1000 psi; more preferably, above about 2000psi; even more preferably above about 3000 psi; and most preferablyabove about 5000 psi without premature failure. Also preferably, thepressure sensor is capable of measuring pressures less than about 300psi; more preferably, less than about 200 psi; even more preferably lessthan about 100 psi; and most preferably less than about 75 psi.

Referring now to FIG. 1, hysteresis curves of Electrical Resistance orLength Change or Volume Change vs. Temperature for a hypothetical SMAmaterial is shown for three different levels of strain, i.e., zerostrain, moderate strain, and high strain. In each instance, the bottomportion of each curve represents the material when in the martensitephase and the top portion of each curve represents the material when inthe austenite phase. The portions of each curve interconnecting thebottom portion of the curve with the top portion of the curve representsa phase transformation, i.e., either from the martensite phase to theaustenite phase or from the austenite phase to the martensite phase. Ascan be seen, each curve has a similar shape and as strain increases, thecurves shift in a positive direction along the X and Y axes. Uponheating, the SMA material spontaneously transforms from the martensitephase to the austenite phase at the phase transformation temperature(the velocity of transformation is the acoustic velocity). The phasetransformation temperature is a relatively narrow band of temperatures.Many of the physical properties of an SMA material, including electricalresistance, ductility, Young's Modulus, reflectivity, etc., undergo asubstantial change in value during a phase transformation. The presentinvention relates to the large change in electrical resistance thatoccurs in SMA material during a phase transformation.

It has been found that when an SMA material is held at or near its phasetransformation temperature, an application of strain to the materialcauses the material to undergo the phase transformation with acorresponding large change in the electrical resistance of the material.The amount of strain required to induce the transformation may be quitesmall, on the order of 0.1% or less. The figure of merit for straingages is called the gage factor and is defined as the normalized changein electrical resistance divided by the change in strain (GageFactor=G.F.=((ΔR/R)/ε), where R is the electrical resistance; ΔR is thechange in the electrical resistance; and ε is the strain. The gagefactor for typical metal film strain gages is on the order of 2 to 5.Silicon and polycrystalline silicon piezoresistors have gage factorsthat vary from less than 1 to over 100 depending upon their orientation,doping level, crystalline perfection, and the temperature ofapplication. However, this level of gage factor is quite difficult toachieve in practice. Furthermore, the high gage factor of siliconmaterials is lost when operated at elevated temperatures. The change inthe electrical resistance of SMA material at its phase transformationtemperature can, however, be on the order of 20% for a strain of 0.1%,thereby achieving a gage factor of nearly 200 (ΔR/R=0.2 and ε=0.001;therefore, 0.2/0.001=200). Also, SMA materials can have phasetransformation temperatures in excess of 550° C. and, therefore, can beutilized as highly sensitive strain sensors at elevated temperature. Thepresent invention discloses a method and a sensor made from SMA materialto utilize this effect.

Preferably, the pressure sensor of the present invention has a gagefactor of at least about 35 at temperatures of at least about 37° C.,more preferably a gage factor of at least about 40 at temperatures of atleast about 37° C., and most preferably a gage factor of at least about42 at temperatures of at least about 37° C. Preferably, the pressuresensor of the present invention has a gage factor of at least about 27at temperatures of at least about 200° C., more preferably a gage factorof at least about 32 at temperatures of at least about 200° C., and mostpreferably a gage factor of at least about 37 at temperatures of atleast about 200° C. Preferably, the pressure sensor of the presentinvention has a gage factor of at least about 22 at temperatures of atleast about 400° C., more preferably a gage factor of at least about 30at temperatures of at least about 400° C., and most preferably a gagefactor of at least about 35 at temperatures of at least about 400° C.Preferably, the pressure sensor of the present invention has a gagefactor of at least about 20 at temperatures of at least about 500° C.,more preferably a gage factor of at least about 30 at temperatures of atleast about 500° C., and most preferably a gage factor of at least about35 at temperatures of at least about 500° C. Preferably, the pressuresensor of the present invention has a gage factor of at least about 16at temperatures of at least about 550° C., more preferably a gage factorof at least about 25 at temperatures of at least about 550° C., evenmore preferably a gage factor of at least about 30 at temperatures of atleast about 550° C. and most preferably a gage factor of at least about35 at temperatures of at least about 550° C.

In another embodiment of the present invention, preferably the pressuresensor has a heating element capable of heating the sensing element,whether it be a strain gage or some other type of element, to at leastabout the application temperature (and in the case of SMA materials tothe transformation temperature). This allows for stabilization of thesensing element in applications such as engine applications where thetemperature varies. This also provides a method of determining pressuresin an engine comprising the steps of providing a sensing element for achamber having a given operating temperature for the chamber in which apressure is to be measured, heating the sensing element to at leastabout the operating temperature of the chamber and measuring thepressure of the chamber through the sensing element. Preferably, theheating element heats the sensing element to a temperature at or abovethe highest application temperature of the sensing element, if thetemperature is cyclical at or near the maximum application temperature.Several embodiments of the heating element are more specificallydescribed later in the application in reference to FIGS. 7, 8, 9, 10, 11and 12.

The heater element and pressure-sensing element of the pressure sensormay be integrated together onto the pressure sensor diaphragm; theheater element and the pressure-sensing element being configured toprevent short circuiting between the heating and pressure-sensingelements, and to allow the heater to maintain stable thermalcharacteristics of the pressure-sensing element, preferably similar tothe application environment. Depending on the characteristics of thematerial, thickness and shape of the diaphragm, or the application, theheater can be positioned on the opposite side of the diaphragm from thesensing element, positioned adjacent to the sensing element, positionedabove or below the sensing element (but separated by a dielectric layer)or in any other position relative to the sensing element that providesstable thermal characteristics during the application or use of thepressure sensor.

The heater element can be controlled to maintain a stable thermalenvironment, with temperatures at or around the application temperature,for the pressure-sensing element using another sensor to sense thetemperature and a controller to receive signals from this sensor andadjust the electrical input into the heater element to achieve ormaintain thermal stability about the pressure-sensing element.Alternatively, with the proper heater element having thermally sensitiveresistive characteristics, the temperature at or about the heaterelement can be determined by the electrical requirement characteristicsof the heater element to determine the temperature and thereby through acontroller to maintain the thermal stability about the pressure-sensingelement. Preferably, the heating element is a resistive heater. Morepreferably, the heating element is made in part from one or more of thefollowing materials. For example, platinum, gold, and nichrome, whichare stable materials at the application temperatures of the pressuresensor. In addition, non-metallic materials will also work such aspolycrystalline silicon.

The SMA material of certain specific embodiments of the presentinvention can be, but is not limited to, binary and equal parts (atomicpercent) of elements, binary and unequal parts of elements, or ternaryor quaternary parts of various compositions of elements. Thesecompositions may comprise elements such as a mixture of titanium andnickel (TiNi) or titanium, nickel and palladium (TiNiPd) although it canbe appreciated by one having ordinary skill in the art that the presentinvention is not limited to SMA material comprised of the aforementionedelements. Variations in composition and alloying content affect thetemperature at which a phase transformation occurs. For example, in aSMA material comprising TiNi having approximately 50% atomic weight ofeach element, a 1 to 2% change in the percentage of titanium to nickelshifts the phase transformation temperature from below 0 to over 90 C.Thus, the phase transformation temperature can be compositionallytailored by utilizing binary alloys and can be extended by using ternaryalloys. An SMA material comprising TiNiPd can have a phasetransformation temperature as high as 550 to 600 C depending upon therelative concentration of Pd to Ni. As Pd is substituted for Ni, thephase transformation temperature generally increases until the resultingcompound is completely TiPd whereupon the phase transformationtemperature is at its maximum.

Referring now to FIG. 2, there is shown a graph of Electrical Resistancevs. Temperature illustrating the hysteresis curves of SMA material understrain levels of 0.12% 12, 0.23% 14 and 0.35% 16. Each hysteresis curvehas an austenite start point 121, 141 and 161; an austenite finishpoint, 122, 142 and 162; a martensite start point 123, 143 and 163; anda martensite finish point 124, 144 and 164, defining individualhysteresis curves. As the temperature of the SMA material increases, itreaches the austenite start point 121, 141, 161 and the austenite phasetransformation begins. The electrical resistance of the materialdecreases until it reaches its austenite finish point 122, 142, 162. Asthe temperature of the SMA material is then decreased, the materialreaches its martensite start point 123, 143 and 163 and the martensitephase transformation begins. The electrical resistance of the materialincreases until the material reaches its martensite finish point 124,134 and 164. As is evident from the graph, the hysteresis curves shiftin response to changing strain, generally moving in a positive directionwith respect to both the X and Y axes in response to increasing strain.This “shifting” characteristic causes the electrical resistance of theSMA material to change with respect to both temperature and strain.

In one application of the present invention, the SMA material is heatedto its austenite start point and then maintained at that temperature. Asthe strain increases, the electrical resistance of the SMA material atthe austenite start point is measured. In FIG. 2, at approximately 450°C., this electrical resistance is 4.78 ohms for 0.12 strain 121, 5.04ohms for 0.23% strain, and 5.12 ohms for 0.35% strain. In anotherapplication of the present invention, the SMA material is heated pastits austenite phase transformation point, and then cooled to itsmartensite start point and maintained at that temperature. As the strainincreases, the electrical resistance of the SMA material at themartensite start point is measured, In FIG. 2, at approximately 600° C.,this electrical resistance is 4.28 ohms for 0.12% strain 123, 4.43 ohmsfor 0.23% strain and 4.46 ohms for 0.35% strain. In still anotherapplication of the present invention, the SMA material is heated andsubsequently cooled through its entire hysteresis curve whilemaintaining strain substantially constant. The characteristics of theresulting curve are compared to other hysteresis curves in a “look-up”table to determine the value of the average strain being applied to theSMA material.

Referring now to FIG. 3, there is shown a graph of Electrical Resistancevs. Loading Condition for a thin film or wire SMA material subjected toan increasing strain level from 0-1.2%. As can be seen from this graph,the electrical resistance of the thin film SMA material increases in asubstantially linear manner, from 5.49 ohms to 5.95 ohms, with anincrease in the level of strain from 0.12% to 1.16%. Also, it can beseen that the electrical resistance of the material returns to nearlythe same value (approximately 5.49 ohms) when the strain is removed. Theloading condition corresponds to pressure or force applied to the thinfilm SMA material which produces the strain thereon.

FIG. 4 is a Best Fit Line Graph of Electrical Resistance vs. Strain.This graph illustrates the substantially linear response of the thinfilm or wire SMA material to the application of strain applied thereto.The Best Fit Line is defined by the equation y=45.82x+5.41.

Referring now to FIG. 5, there is shown a plan view of a SMA materialstrain sensor 40. A sensor element 42 formed from thin film TiNimaterial is deposited over a flexible diaphragm 44 on a substrate 46.Typically, the flexible diaphragm 44 has an area of approximately 1 mm²whereas the substrate 46 has an area of approximately 1 cm². Sensorterminals 48 a and 48 b provide electrical connection points for leads(not shown) for attachment of the sensor element 42 to externalmeasuring devices or controllers. When a strain is applied to theflexible diaphragm 44, the sensor element 42 flexes. Since the sensorelement 42 is at the martensite/austenite phase transformationtemperature, the sensor element 42 readily flexes at the applicationtemperature and exhibits substantially linear electrical resistance vs.strain characteristics. The electrical resistance of the sensor element42 changes as the strain applied thereto increases. The electricalresistance of the sensor element 42 can be transmitted through thesensor terminals 48 a and 48 b to external measuring devices orcontrollers. For example, a change in the electrical resistance of thesensor element 42 can be transformed into a change in the voltage acrosssame. In view of the foregoing, the SMA strain sensor 40 can be utilizedin a control circuit where a change in pressure or force is beingmonitored. An example of such a circuit is illustrated in FIG. 7 whichis an electrical schematic of four SMA elements 40 a, 40 b, 40 c, and 40d connected in a Wheatstone Bridge circuit. Because of their electricalresistive characteristics, the SMA elements 40 a, 40 b, 40 c and 40 dcan be utilized in any Wheatstone Bridge circuit application in whichthe change in output voltage corresponds to change in strain.

Referring now to FIG. 8, there is shown a plan view of the SMA materialstrain sensor 40 illustrated in FIG. 4 but further including atemperature measuring element 52 and a heating element 54. Thetemperature measuring element 52 and heating element 54 are used toensure that the temperature of the sensor element 42 is maintained atthe phase transformation temperature. The temperature measuring element52 can be any suitable temperature measuring device whereas the heatingelement 54 can be a resistance heater integrated into the sensor element42, or can be separate there from. The temperature measuring element 52is located on or near the flexible diaphragm 44 to provide an accuratemeasurement of the temperature of the sensor element 42. Typically, theflexible diaphragm 44 is a thin diaphragm resulting in low powerconsumption and fast thermal response. Temperature terminals 56 a and 56b provide a connection between the temperature measuring element 52 andexternal temperature measuring devices. The heating element 54 islocated on the diaphragm 44 to provide a substantially uniformtemperature to the sensor element 42. The heating element 54 is alsocapable of varying the temperature of the sensor element 42 through thephase transformation process. Heater terminals 58 a and 58 b provideconnection between the heater element 54 and an external power sourcewhen the environment is cooler than the phase transformationtemperature. In this manner, the sensor 40 can be operated above ambienttemperature and the sensor operation can be tailored for optimumsensitivity.

Referring now to FIGS. 9 a) and b), in 9 a) there is shown a plan viewof another embodiment of the pressure-sensing element 42 and the heatingelement 54 and in 9 b) is a cross-sectional view. The diaphragm 44 beingpart of the overall sensor substrate 90. In this particular embodiment,the diaphragm or flexible diaphragm 44, has an upper 43 and a lower 45surface. The pressure-sensing element 42 and the heating element 54both, in part, placed on the upper surface 43 of the diaphragm. Theheating element 54 is used to ensure that the temperature of the sensorelement 42 is maintained at about the temperatures that characterize thephase transformation. The sensor substrate optionally contains atemperature measuring element (not shown), which can be any suitabletemperature measuring device whereas the heating element 54 preferablyis a resistive heater integrated into the sensor element 42, or can beseparate there from. The temperature measuring element (not shown), ifused, is preferably located on or near the flexible diaphragm 44 toprovide an accurate measurement about the area of the sensor element 42.Typically, the flexible diaphragm 44 is a thin diaphragm resulting inlow power consumption and fast thermal response. The heating element 54is located on the diaphragm 44 to provide a substantially uniformtemperature to the sensor element 42. The heating element 54 is alsocapable of varying the temperature of the sensor element 42 through thephase transformation process. In this manner, the sensor 40 can beoperated above ambient temperature and the sensor operation can betailored for optimum sensitivity.

Referring now to FIGS. 10 a) and b), in 10 a) there is shown a plan viewof another embodiment of the pressure-sensing element 42 and the heatingelement 54 and in 10 b) is a cross-sectional view. The diaphragm 44being part of the overall sensor substrate 90. In this particularembodiment, the diaphragm or flexible diaphragm 44, has an upper 43 anda lower 45 surface. In this particular embodiment, the heating element54 is positioned directly on the upper surface 43 of the sensorsubstrate 90 over which a dielectric layer 91 is incorporated. Thedielectric layer 91 is used to electrically separate the heating element54 from the pressure-sensing element 42. The pressure-sensing element 42in this embodiment, in part, overlaps the heating element 54 and isincorporated over the dielectric layer 91. The dielectric layer 91 inthis and other embodiments preferably has good thermal conductivityproperties. Again, in this embodiment also the heating element 54 ispreferably used to ensure that the temperature of the sensor element 42is maintained at or about the phase transformation temperature. Thesensor substrate 90 optionally contains a temperature measuring element(not shown), which can be any suitable temperature measuring device. Theheating element 54 preferably is a resistive heater integrated into thesensor element 42, or can be separate there from. The temperaturemeasuring element (not shown), if used, is preferably located on or nearthe flexible diaphragm 44 and in close proximity to the pressure-sensingelement 54 so as to provide an accurate measurement about the area ofthe sensor element 42. Typically, the flexible diaphragm 44 is a thindiaphragm resulting in low power consumption and fast thermal response.The heating element 54 is preferably located on the diaphragm 44 toprovide a substantially uniform temperature to the sensor element 42.The heating element 54 is also capable of varying the temperature of thesensor element 42 through the phase transformation process. In thismanner, the sensor 40 can be operated above ambient temperature and thesensor operation can be tailored for optimum sensitivity.

Referring now to FIGS. 11 a) and b), in 11 a) there is shown a plan viewof another embodiment of the pressure-sensing element 42 and the heatingelement 54 and in 11 b) is a cross-sectional view. The diaphragm 44being part of the overall sensor substrate 90. In this particularembodiment, the sensor substrate 90 and the diaphragm or flexiblediaphragm 44, has an upper 43 and a lower 45 surface. In this particularembodiment, a dielectric layer 91 is formed on the upper surface of thesensor substrate 90. The heating element 54 is then positioned in partover the dielectric layer 91. The dielectric layer 91 is used toelectrically separate the heating element 54 from the pressure-sensingelement 42. The pressure-sensing element 42 in this particularembodiment is applied directly to the top surface 43 of the sensorsubstrate 90. The dielectric layer 91 in this and other embodimentspreferably has good thermal conductivity properties. Again, in thisembodiment as well, the heating element 54 is preferably used to ensurethat the temperature of the sensor element 42 is maintained at or aboutthe phase transformation temperature. The sensor substrate 90 optionallycontains a temperature measuring element (not shown), which can be anysuitable temperature measuring device. The heating element 54 preferablyis a resistive heater integrated into the sensor element 42, or can beseparate there from. The temperature measuring element (not shown) ifused is preferably located on or near the flexible diaphragm 44 and inclose proximity to the pressure-sensing element 54 so as to provide anaccurate measurement about the area of the sensor element 42. Typically,the flexible diaphragm 44 is a thin diaphragm resulting in low powerconsumption and fast thermal response. The heating element 54 ispreferably located on the diaphragm 44 to provide a substantiallyuniform temperature to the sensor element 42. The heating element 54 isalso capable of varying the temperature of the sensor element 42 throughthe phase transformation process. In this manner, the pressure-sensingelement 42 can be operated above ambient temperature and the sensoroperation can be tailored for optimum sensitivity.

Referring now to FIGS. 12 a), b), and c), in 12 a) there is shown a planview of the top surface another embodiment of the pressure-sensingelement 42, in 12 b) is a cross-sectional view, and in 12 c) is shown aplan view of the bottom surface of the substrate 90 holding thepressure-sensing element 42. The diaphragm 44 being part of the overallsensor substrate 90. In this particular embodiment, the sensor substrate90 and the diaphragm or flexible diaphragm 44, has an upper 43 and alower 45 surface. In this particular embodiment, the pressure-sensingelement 42 is located on the top surface 43 of the substrate 90. Theheating element 54 is located on the bottom surface 45 of the substrate90. The pressure-sensing element 42 is separated from the heatingelement 54 by the substrate, which prevents any electrical shortcircuiting between the two elements. The substrate in this embodimentpreferably has good thermal conductivity properties. Again, in thisembodiment also the heating element 54 is preferably used to ensure thatthe temperature of the sensor element 42 is maintained at or about thephase transformation temperature. The sensor substrate 90 optionallycontains a temperature measuring element (not shown), which can be anysuitable temperature measuring device. The heating element 54 preferablyis a resistive heater integrated into the sensor element 42, or can beseparate there from. The temperature measuring element (not shown), ifused, is preferably located on or near the flexible diaphragm 44 and inclose proximity to the pressure-sensing element 54 so as to provide anaccurate measurement about the area of the pressure-sensing element 42.Typically, the flexible diaphragm 44 is a thin diaphragm resulting inlow power consumption and fast thermal response. The heating element 54is preferably located on the diaphragm 44 to provide a substantiallyuniform temperature to the sensor element 42. The heating element 54 isalso capable of varying the temperature of the sensor element 42 throughthe phase transformation process. In this manner, the pressure-sensingelement 42 can be operated above ambient temperature and the sensoroperation can be tailored for optimum sensitivity.

Referring now to FIG. 13, there is shown a cross-section of a portion ofa pressure sensor where the heating element is not incorporated on thesame substrate as the heating element. In FIG. 13, the pressure-sensingelement 42 is incorporated on the top surface of the sensor substrate90. The diaphragm 44 being part of the overall sensor substrate 90. Theheating element 54 in this embodiment is incorporated on the lowersurface 93 of a separate heater substrate 92. The heater substrate 92 isthen packaged with the sensor substrate 90 by placing the heatingelement 54 onto the top surface 90 of the sensor substrate making surethat the heating element 54 and the pressure-sensing element 42 do notoverlap and that there is enough distance between the two elements sothat little or no electrical crossover occurs between the elements. Theflexible diaphragm 44 is a thin diaphragm resulting in low powerconsumption and fast thermal response. The heating element 54 ispreferably located in close proximity to the diaphragm 44 to provide asubstantially uniform temperature to the sensor element 42. The heatingelement 54 is also capable of varying the temperature of the sensorelement 42 through the phase transformation process. In this manner, thepressure-sensing element 42 can be operated above ambient temperatureand the sensor operation can be tailored for optimum sensitivity.

Referring now to FIG. 14, there is shown a cross-section of a portion ofa pressure sensor where the heating element is not incorporated on thesame substrate as the heating element. In FIG. 14, the pressure-sensingelement 42 is incorporated on the top surface of the sensor substrate90. The diaphragm 44 being part of the overall sensor substrate 90. Theheating element 54 in this embodiment is incorporated on the uppersurface 95 of a separate heater substrate 92. The heater substrate 92 isthen packaged with the sensor substrate 90 by placing the lower surface93 of the heater substrate 92 in contact with the top surface 90 of thesensor substrate making sure that the heating element 54 is in closeproximity to the pressure-sensing element 42 to allow low powerconsumption and fast thermal response with the heating element 54, andproviding a substantially uniform temperature to the sensor element 42.The heating element 54 is also capable of varying the temperature of thesensor element 42 through the phase transformation process. In thismanner, the pressure-sensing element 42 can be operated above ambienttemperature and the sensor operation can be tailored for optimumsensitivity.

Referring now to FIG. 15, there is a block diagram of one embodiment ofthe electrical circuitry for operating the temperature control andmeasurement with the bi-functional sensing element. In FIG. 15, thePulse Width Modulator (PWM) 10 adjusts the duty cycle of the powerswitch 112 to increase or decrease power from the power supply 116 tothe heating element 54. The resistance is measured 114 during the “off”portion of the duty cycle, and this value adjusts the PWM 110 for thenext cycle.

Referring now to FIG. 16, there is a block diagram of another embodimentof the electrical circuitry for operating the temperature control andmeasurement with the bi-functional heating element. In FIG. 16, theresistance monitor 120 continuously measures the heating element 54 forthe resistance corresponding to the desired temperature. Deviationsabove that resistance/temperature point will decrease the current 122 tothe heating element 54. The opposite occurs for deviations above thedesired resistance/temperature point.

Referring now to FIG. 17, there is a cut away schematic illustration ofa portion of the pressure sensor of the present invention mounted in apressure chamber where heat flux is into the sensing element from thechamber. In FIG. 17, the pressure sensor head 140 is mounted into apressure chamber 132 so that the lower surface 45 of the sensorsubstrate 90 is exposed to the inside of the chamber 132. In thisparticular embodiment, the heating element 54 is mounted on the lowersurface 45 of the sensor substrate 90 and the pressure-sensing element(not shown) is mounted on the upper surface (not shown) of the flexiblediaphragm 44. Further shown in this Figure are two leads 130 from theheating element 54, which are protected by the packaging sheath 136 ofthe pressure sensor 142. Additionally, there are leads 138 from thepressure-sensing element (not shown), which are also protected by thepackaging sheath 136. The curvy lines 134 represent the heat flux fromthe pressure chamber 132 into the pressure sensor head 140.

Referring now to FIG. 18, there is a cut away schematic illustration ofa portion of the pressure sensor of the present invention mounted in apressure chamber where heat flux is into the chamber from the sensingelement. In FIG. 18, the pressure sensor head 140 is mounted into apressure chamber 132 so that the lower surface 45 of the sensorsubstrate 90 is exposed to the inside of the chamber 132. In thisparticular embodiment, the heating element 54 is mounted on the lowersurface 45 of the sensor substrate 90 and the pressure-sensing element(not shown) is mounted on the upper surface (not shown) of the flexiblediaphragm 44. Further shown in this Figure are two leads 130 from theheating element 54, which are protected by the packaging sheath 136 ofthe pressure sensor 142. Additionally, there are leads 138 from thepressure-sensing element (not shown), which are also protected by thepackaging sheath 136. The curvy lines 150 represent the heat flux fromthe pressure chamber 132 into the pressure sensor head 140.

Referring now to FIG. 19, there is a graph comparing the gage factors ofother prior art pressure sensors with the pressure sensor of the presentinvention over a wide temperature range. The line 80 in the graphrepresents the gage factors of metal film strain gage pressure sensors.As can be seen in the graph, metal film pressure sensors have a low gagefactor across the entire temperature range of from approximately 50-550°C. The lines 82 and 84 represent n-type and p-type piezoresistivepressure sensors. As can be seen in the graph, these types of pressuresensors can have higher gage factors at the lower temperatures, but thegage factor quickly degrades with increasing temperature becoming lowerthan that of a metal film pressure sensor at temperatures above 550° C.The line 86 represents a β-silicon carbide pressure sensor. While thispressure sensor exhibits an improved gage factor at elevatedtemperatures, the gage factor is still very limiting from the aspect ofdesigning the overall sensor. Finally, line 88 represents the pressuresensor of the present invention. This pressure sensor exhibits anexcellent gage factor across the entire temperature range and allows forthe design of smaller more sensitive pressure sensors.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A pressure sensor comprising a strain gauge; a diaphragm having anupper and a lower surface, the strain gauge on the upper surface of thediaphragm; and a cooling element capable of cooling the strain gauge andmeasuring a temperature about the cooling element.
 2. The pressuresensor in claim 1, wherein the strain gauge is made from a shape memoryalloy.
 3. The pressure sensor in claim 1, wherein the cooling element iscapable of maintaining the sensing element at a temperature at or belowan application temperature of the pressure sensor.
 4. The pressuresensor in claim 3, wherein the application temperature is at least about200° C.
 5. The pressure sensor in claim 3, wherein the applicationtemperature is at least about 400° C.
 6. The pressure sensor in claim 1,wherein the pressure sensor is used in a diesel engine.
 7. A pressuresensor comprising a sensing element; a diaphragm having an upper and alower surface; and a cooling element wherein the cooling elementmaintains a temperature of the sensing element at or below an operatingtemperature range of the pressure sensor and is used to measure theoperating temperature of the pressure sensor.
 8. The pressure sensor inclaim 7, wherein the sensing element is made from a shape memory alloy.9. The pressure sensor in claim 7, wherein the application temperatureis at least about 200° C.
 10. The pressure sensor in claim 7, whereinthe application temperature is at least about 400° C.
 11. The pressuresensor in claim 7, wherein the pressure sensor is used in a dieselengine.
 12. The pressure sensor in claim 7, wherein the pressure sensoris used in a turbine engine.
 13. The pressure sensor in claim 7, whereinthe pressure sensor is capable of measuring pressures above 1000 psiwithout premature failure.
 14. The pressure sensor in claim 7, whereinthe pressure sensor is capable of measuring pressures above 3000 psiwithout premature failure.
 15. A method of controlling a temperatureabout a pressure-sensing element comprising the steps of: providing apressure-sensing element for an elevated temperature application inwhich a pressure is to be measured; providing a cooling element; coolingthe pressure-sensing element, at least in part, with the coolingelement; determining or estimating the temperature of about thepressure-sensing element by measuring at least one electricalcharacteristic of the cooling element; and adjusting the cooling elementin response to the measured electrical characteristic or characteristicsof the cooling element.
 16. The method in claim 15, wherein thepressure-sensing element has a application temperature of at least about200° C.
 17. The method in claim 15, wherein the pressure-sensing elementhas a application temperature of at least about 400° C.
 18. The methodin claim 15, wherein the pressure-sensing element is manufactured from ashape memory alloy.
 19. The method in claim 15, wherein thepressure-sensing element is cooled to at or below the applicationtemperature of the pressure sensor.
 20. The method in claim 15, whereinthe elevated temperature application is measuring the pressure in anengine.